# Using a Base.CFunction in ccall
**URL:** https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621
**Category:** General Usage
**Created:** [August 17, 2018, 1:15pm UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621 "2018-08-17T13:15:38Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@giordano](https://discourse.julialang.org/u/giordano)
#### Post date: [August 17, 2018, 1:15pm UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/1 "2018-08-17T13:15:38Z")
I want to do something like this, inside a `begin` block:
```julia
begin
f(x) = x * x
ptr = @cfunction(f, Cdouble, (Cdouble,))
g(x) = ccall(ptr, Cdouble, (Cdouble,), x)
g(1.2)
end
```
However this gives the following error:
```julia
ERROR: LoadError: UndefVarError: f not defined
```
I’ve been suggested by @kristoffer.carlsson to use `$f` in `@cfunction`, thus resulting in the creation of `Base.CFunction`. However, I don’t understand ho to use in a `ccall`. I couldn’t find any such example in Julia’s `Base`. `Base.CFunction`’s docstring reads:
> Like all `cfunction` handles, it should be passed to `ccall` as a `Ptr{Cvoid}`, and will be converted automatically at the call site to the appropriate type.
How to convert a `Base.CFunction` to `Ptr{Cvoid}`? `convert` and `Base.cconvert` don’t work, but `Base.unsafe_convert` may do the trick. Is this the way to go (i.e., `ccall(Base.unsafe_convert(Ptr{Cvoid}, ptr), ...)`)?
---
### Author:  [@giordano](https://discourse.julialang.org/u/giordano)
#### Post date: [August 30, 2018, 9:46pm UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/2 "2018-08-30T21:46:41Z")
Does anyone have a clue about how to do this?
---
### Author:  [@cdsousa](https://discourse.julialang.org/u/cdsousa)
#### Post date: [August 30, 2018, 11:42pm UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/3 "2018-08-30T23:42:42Z")
Is there some problem due to be inside the begin end block?
Anyway, just to confirm, are you aware of the qsort example [https://docs.julialang.org/en/stable/manual/calling-c-and-fortran-code/#Creating-C-Compatible-Julia-Function-Pointers-1](https://docs.julialang.org/en/stable/manual/calling-c-and-fortran-code/#Creating-C-Compatible-Julia-Function-Pointers-1), right?
---
### Author:  [@giordano](https://discourse.julialang.org/u/giordano)
#### Post date: [August 31, 2018, 9:25am UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/4 "2018-08-31T09:25:37Z")
> [@cdsousa](#):
>
> Is there some problem due to be inside the begin end block?
Yes, I need to do this inside a begin-end block, specifically it’s in a `@testset`.
> [@cdsousa](#):
>
> Anyway, just to confirm, are you aware of the qsort example [https://docs.julialang.org/en/stable/manual/calling-c-and-fortran-code/#Creating-C-Compatible-Julia-Function-Pointers-1](https://docs.julialang.org/en/stable/manual/calling-c-and-fortran-code/#Creating-C-Compatible-Julia-Function-Pointers-1), right?
Yes, I know that example and I’ve used callbacks in `Cuba.jl`. However, here there is no callback, I just want to `ccall` a very simple function defined with `@cfunction`
---
### Author:  [@cdsousa](https://discourse.julialang.org/u/cdsousa)
#### Post date: [August 31, 2018, 1:41pm UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/5 "2018-08-31T13:41:41Z")
Oh, that’s true, I had not read your original question very carefully.
---
### Author:  [@giordano](https://discourse.julialang.org/u/giordano)
#### Post date: [November 5, 2018, 11:06pm UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/6 "2018-11-05T23:06:38Z")
Bump. Does anyone know how to do this?
---
### Author:  [@JeffreySarnoff](https://discourse.julialang.org/u/JeffreySarnoff)
#### Post date: [November 6, 2018, 12:26am UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/7 "2018-11-06T00:26:09Z")
```julia
(edited, see next comment for the correct solution)
julia> function f end
begin
global f
f(x) = x * x
ptr = @cfunction(f, Cdouble, (Cdouble,))
g(x) = ccall(ptr, Cdouble, (Cdouble,), x)
g(1.2)
end
1.44
```
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [November 6, 2018, 12:55am UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/8 "2018-11-06T00:55:36Z")
No that’s not a closure (If you don’t need a closure then that’s fine but in that case you should use `function f end` instead of defining a dummy method).
```julia
julia> let f = (x)->x * x
ptr = @cfunction $f Cdouble (Cdouble,)
GC.@preserve ptr ccall(Base.unsafe_convert(Ptr{Cvoid}, ptr), Cdouble, (Cdouble,), 2.0)
end
4.0
```
---
### Author:  [@giordano](https://discourse.julialang.org/u/giordano)
#### Post date: [November 6, 2018, 6:04pm UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/9 "2018-11-06T18:04:06Z")
Ok, so `Base.unsafe_convert` was indeed the way to go, I was missing the `GC.@preserve`. Thank you!
---
### Author:  [@system](https://discourse.julialang.org/u/system)
#### Post date: [November 18, 2018, 6:04pm UTC](https://discourse.julialang.org/t/using-a-base-cfunction-in-ccall/13621/10 "2018-11-18T18:04:12Z")
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